FERC Project No. 2079 - PCWA Middle Fork American River Project ...

Application for New License Middle Fork American River Project (FERC Project No. 2079)
FINE SEDIMENT IN POOLS AND GRAVELS
In the Middle Fork American and Rubicon rivers, the Proposed Action
maintains/schedules a similar frequency of high-flow events (5-year recurrence and
gravel initiation of motion flows) that has occurred historically in the No-Action
Alternative 4
(Exhibit E – Section 8.7, Tables 8.7-3a and 8.7-4a). Therefore, fine
sediment in pools and spawning gravels will be maintained at the level of existing
conditions (No-Action Alternative). Under existing conditions, the fine sediment volume
in pools was very low (less than 8% and often much less) (Exhibit E – Section 8.7,
Table 8.7-2) and the percentage of fine sediments in spawning gravels was low and
conducive to high spawning success (Exhibit E – Section 7.7, Geomorphology
Resources Affected Environment) in all river reaches (Exhibit E – Section 7.7,
Table 7.7-8).
In the small streams (Duncan, North Fork Long Canyon, South Fork Long Canyon, and
mainstem Long Canyon creeks), the number days and years that the 5-year recurrence
flows and the gravel initiation of motion flows occur under the Proposed Action are
reduced when compared to the existing conditions (No-Action Alternative) (Exhibit E –
Section 8.7, Tables 8.7-3b and 8.7-4b). For example, the number of days of initiation
motion in the Proposed Action is 50–80% of days in the No-Action Alternative.
However, the number of days/years these flow events occur in the Proposed Action is
sufficient to maintain low fine sediment content in pools and spawning gravels
equivalent to the No-Action Alternative. In part, this condition is due to the low fine
sediment supply present in the system (also see discussion below). The GRMP (PCWA
2011q; SD A) in the Proposed Action will be used to document fine sediment conditions
in the small streams. A report summarizing the data collected each monitoring period
will be prepared by PCWA and distributed to the USDA-FS, State Water Board, and
CDFG for review and comment. Based on the results of the monitoring and/or
comments received during the review process, PCWA and the agencies may call a
meeting to discuss the results.
GRAVEL SUPPLY
The Proposed Action specifies modifications to the small diversions and sediment
augmentation activities at the medium reservoirs (Middle Fork Interbay and Ralston
Afterbay) that will enhance sediment supply, and particularly gravels, in five bypass
reaches compared to the No-Action Alternative. Gravel supply would increase
downstream of the following facilities: Duncan Creek Diversion, North Fork Long
Canyon Creek Diversion, South Fork Long Canyon Diversion, Middle Fork Interbay, and
Ralston Afterbay. Exhibit E – Section 7.7, Table 7.7-11 shows the estimated average
annual amount of spawning gravel-sized sediment (75 to 2,645 cubic yards/year
depending on location) that will pass downstream of these facilities in the Proposed
4 The No-Action Alternative does not include specified channel maintenance flows and while they have
occurred in past operations, it is not guaranteed that they would continue in the future under the No-
Action Alternative.
February 2011 6-17